Biocatalyst-immobilized electrode and method for treatment of water by use of the electrode

ABSTRACT

An electrode consists of a cathode matrix material and a biocatalyst immobilized on the matrix material. A water under treatment is treated by disposing the electrode as a cathode and an anode opposed thereto in the water, and applying an electric current between the cathode and the anode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a biocatalyst-immobilized electrode for use ina bioreactor which is useful for producing and recovering usefulsubstances and decomposing environment-polluting substances in water byenhancing the biochemical reaction of a biocatalyst such as a microbialcell body or an enzyme, the electrochemical reaction on the surface ofthe electrode, and the biochemical reaction of the biocatalyst by theelectric current and to a method for the treatment of water by use ofthe biocatalyst-immobilized electrode.

2. Prior Art Statement

The method which decomposes and removes organic substances and inorganicions present in water under treatment by feeding the water to a treatingcolumn containing an aerobic microbe or anaerobic microbe as abiocatalyst and supplying a substrate (an organic substance as ahydrogen donor or hydrogen) to the treating column and the method whichelectrolyzes organic substances and inorganic ions present in waterunder treatment by feeding the water to an electrolytic solution inwhich an electrode is present and applying an electric current to theelectrolytic solution are known to the art.

In the former of the methods mentioned above, when organic substancesare supplied, they are required to be in an excess amount forheightening the efficiency of water treatment. This requirement may leadto pollution of the water under treatment by the excess organicsubstances. When hydrogen is supplied, since hydrogen shows only verylow solubility to water and since the reaction proceeds at a rate whichis determined by the supply of hydrogen, polluting substances can bethoroughly removed only with extreme difficulty and there is a largepossibility of the reaction giving rise to an intermediate whichsurvives treatment for removal.

As the latter method resorting to electrolysis may give rise tosecondary products, cannot easily achieve its object.

SUMMARY OF THE INVENTION

This invention has been developed for the purpose of eliminating theproblems: of the prior art mentioned above.

To be specific, this invention is directed to an electrode consistingessentially of a cathode matrix material and an immobilized biocatalystand to a method for treating water by preparing an electrode having abiocatalyst immobilized on a cathode matrix, disposing the electrode asa cathode in the water under treatment, opposing an anode thereto in thewater, and applying an electric current between the cathode and theanode, thereby inducing a biochemical and an electrochemical reaction ofthe catalyst.

The above and other features of the invention will become apparent fromthe following description made with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the relation between the electric currentand the rate of N₂ generation in Test Example 1.

FIG. 2 is a diagram showing the time-course changes in the NO₃ ⁻concentration of the water under treatment and the amount of N₂generation in Test Example 2.

FIG. 3 is a diagram showing the relation between the elapse of time andthe amount of methane generated in Example 2.

FIG. 4 is a diagram showing the relation between the elapse of time andthe amount of methane generated in Example 3.

FIG. 5 is a diagram showing the relation between the elapse of time andthe amounts of methane and hydrogen generated in Example 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The electrode of this invention described above is intended to be usedas a cathode for a bioreactor. The matrix material for the electrode istherefore required to be ideally suited to the cathode, excel inelectroconductivity and durability, and enable a biocatalyst to beeasily immobilized on the surface thereof. A carbonaceous material ispreferred.

Since the material for the electrode is destined to have the biocatalystimmobilized on the surface thereof, the surface of this material isdesired to be rough or porous so as to facilitate the immobilization.Pretreating the material with plasma increases the adherence between thebiocatalyst and the material.

Graphite can be cited as a concrete example of a carbonaceous material.

Though the shape of the electrode is not particularly restricted, a bar,a plate, a film, etc. can be cited as examples of the shape. Thecross-sectional shape of the material can be freely selected and may,for example, be that of the letter Y, of a cross, of a star, etc.

The biocatalyst is desired to be immobilized as entrained in a coveringmaterial so that when the electrode of this invention is put to actualuse, the biocatalyst will not be degraded in activity as by a poisonoussubstance possibly present in the water under treatment. Materialseffectively usable for the covering include dextran, carrageenan,alginic acid and derivatives thereof, polyvinyl alcohol, photo-linkingresin, urethane, and polyacrylamide gel, for example.

Among the covering materials mentioned above, carrageenan and polyvinylalcohol prove to be particularly desirable.

One example of the method for effecting the immobilization with thecovering comprises suspending given microbial cells in an aqueouspolyvinyl alcohol solution, applying the resultant suspension of themicrobial cell body to the surface of the electrode, and draining thewet electrode thereby forming a coating of the microbial cells on theelectrode.

The biocatalyst can also be immobilized through the medium of asupporting member instead of being directly immobilized on the cathodematrix.

Materials effectively usable for the supporting member herein includewoven and non-woven fabrics of natural fibers, synthetic fibers, andcarbon fibers, for example.

Though the material for the anode is not particularly restricted, it canbe carbon, platinum, or nickel, for example.

When the water under treatment contains NO₃ ⁻, the NO₃ ⁻ converted intoN₂ and expelled as such from the water by the following reactions whichare induced by applying an electric current to the water.

    H.sub.2 O+e.sup.- →1/2H.sub.2 +OH.sup.-             (1)

    2NO.sub.3.sup.- +2H.sup.+ +5H.sub.2 →N.sub.2 +6H.sub.2 O (2)

Biocatalysts effectively usable in the present invention includeParacoccus denitrificans, Micrococcus denitrificans, Alcaligenous,Pseudomonas, C. aceticum, A. woodii, Methanobacterium, Enterobactercloacal, and sulfuric acid reductases, for example.

For the immobilization of a biocatalyst in the present invention, therecan, for example, be used the method which comprises immersing a matrixmaterial for immobilization in a slurry containing the biocatalyst, thenadding a substrate in an amount calculated to assume a prescribedconcentration therein, and allowing the matrix material to stand for aprescribed period in the slurry thereby enabling the biocatalyst to beimmobilized on the matrix material and the method which comprisescoating the surface of a matrix material for immobilization with a webof absorbent macromolecular fibers and immersing the coated matrixmaterial in a slurry containing the biocatalyst thereby enabling thebiocatalyst to be immobilized thereon.

When the water is treated for denitrification in the present invention,the pH value of the water rises with the progress of thedenitrification. In this case, by blowing carbon dioxide gas into thewater, the water can be maintained at the level of neutrality whichallows the denitrifying microbe to manifest high activity. The increasein the number of immobilized microbial cells is extremely small under noaddition of an organic substance. Under addition of an organicsubstance, about 20% of the consumed organic substance goes to microbialassimilation.

Other uses of the method of the present invention include the following:

1. Formation of CH₃ COOH from CO₂ end H₂ based on the followingreactions using A. woodii:

    CO.sub.2 +H.sub.2 O→HCO.sub.3.sup.- +H.sup.+

    HCO.sub.3.sup.- +H.sub.2 →CH.sub.3 COOH

2. Conversion of hexavalent chromium, ion into trivalent chromium ionbased on the following reaction using Enterobacter cloacal:

    CrO.sub.4.sup.2- +H.sub.2 →Cr.sup.3+

3. Reduction of sulfuric acid ion based on the following reaction usingsulfuric acid reductase:

    SO.sub.4.sup.2- +H.sub.2 →H.sub.2 S

4. Formation of CH₄ based on the following reaction usingMethanobacterium:

    CO.sub.2 +H.sub.2 O→HCO.sub.2.sup.- +H.sup.+

    HCO.sub.3.sup.- +H.sub.2 →CH.sub.4

The activity of the catalyst-immobilized electrode increases underapplication of an electric current.

For practical purposes, the amount of this current is in the range offrom 0.001 mA to several A.

When the biocatalyst is immobilized in the covered state, thepossibility of the activity of the catalyst being degraded by the waterunder treatment is nil. The covering further prevents microcontaminationof the water caused by the product of metabolism of the biocatalyst.

When an organic substance is added as a hydrogen donor as popularlypracticed, the organic substance is utilized for the propagation of thebiocatalyst. In the case of the treatment for denitrification, forexample, the amount of the organic substance thus added is required tobe about 2.5 times the stoichiometric amount because the denitrificationentails assimilation and metabolism. In the case of the method of thisinvention, when the water under treatment has a high NO₃ ⁻concentration, the addition of a hydrogen donor can accelerate the speedof denitrification. In the present invention, however, since hydrogen issupplied spontaneously (by electrolysis caused by applying an electriccurrent to the water), the amount of a hydrogen donor to be added isequal to the stoichiometric amount at most, with the result that thepropagation of an organic substance is extremely small and no pollutionof the water occurs.

Hydrogen donors desirably used in the present invention from thepractical point of view include alcohols such as methanol, organic acidssuch as acetic acid, and hydrocarbons such as dextrose, for example,besides hydrogen.

In the conventional method of electrolysis by applying an electriccurrent to the water, the hydrogen utilization ratio is low because thehydrogen generated by the cathode is diffused in the water where itreacts with oxygen. In contrast in the present invention, the reactionproceeds smoothly and very efficiently because the hydrolysis of thewater under treatment induces generation of hydrogen on the surface ofthe electrode and consequently allows very efficient supply of hydrogento the biocatalyst. Since this treatment does not require addition of anorganic substance, there is no possibility of the water being pollutedwith an added substance. The treatment can be easily controlled becausethe hydrogen supply rate can be regulated with the electric current.Further, the electric current enhances the activity of the biocatalyst.

TEST EXAMPLE 1 (Relation Between Electric Current and Amount of N₂Generated in NO₃ ⁻ -containing Water)

A biomembrane was formed on the surface of a carbon bar measuring 8 mmin diameter and 200 mm in length and having a surface coarsenessε(ε=width of surface irregularities/diameter of electrode) in the rangeof from 0.01 to 0.1 by immobilizing Paracoccus denitrificans by thefollowing method.

The immobilization was effected by immersing the carbon bar in a slurrycontaining the microbial cells in a concentration of 3,000 mg/liter,adding a substrate (NaNO₃, acetic acid, etc.) to the slurry in an amountcorresponding to a NO₃ ⁻ concentration in the range of from 200 to 300mg/liter, allowing the carbon bar to stand in the slurry for about onemonth thereby forming a biomembrane on the surface of the carbon bar.Thus an biocatalyst immobilized electrode was obtained.

In a reaction column having an inner volume of about 2,000 cc, thecarbon bar having the biocatalyst immobilized as described above was setin place as a cathode and a carbon bar identical in size with thecathode was opposed as an anode to the cathode. To this column, thewater under treatment (containing NO₃ ⁻ in a concentration of 20mg/liter) was supplied at the rate of 3 cc/min.

The water under treatment was kept at a temperature of 30° C. and anelectric voltage was applied between the opposed electrodes and theamount of electric current was varied, to test for the relation betweenthe magnitude of the electric current and the amount of generated N₂.The results of the test are shown by the straight line 1 in FIG. 1. Theline 1 indicates that 1 mol of electrons generated 0.1 mol of N₂.

Test Example 2 (Time-course Changes of NO₃ ⁻ Concentration and Amount ofGenerated N₂)

In the same reaction column as used in Test Example 1, there was placed2,000 cc of water (containing NO₃ ⁻ in a concentration of 10 mg as N perliter). An electric current of 25 mA was passed through the water andthe water was maintained at a fixed temperature of 30° C., to test fortime-course changes in the NO₃ ⁻ content of the water and the amount ofgenerated N₂. The results are shown in FIG. 2. The line 2 in FIG. 2represents the time-course change of NO₃ ⁻ content and the line 3 theaccumulated amount of generated N₂.

EXAMPLE 1

By the same means as shown in Test Example 1, a biomembrane containing adenitrifying microbe was formed in a thickness of about 100 μm on thesurface of a carbon bar measuring 8 mm in diameter and 20 cm in lengthto produce a biocatalyst-immobilized electrode contemplated by thisinvention.

In a reaction column having an inner volume of about 200 cc, theelectrode mentioned above was set in place as a cathode and an anodeseparately made of carbon was opposed as a counter electrode to thecathode.

A synthetic effluent (containing NaNO₃ in a concentration of about 20mg/liter) was supplied at the rate of 3.3 cc per minute to the reactioncolumn and an electric current was applied to the opposed electrodes inan amount corresponding to a cathode current density of 0.019 mA/cm².The reaction column was operated continuously for 120 hours under theconditions mentioned above.

The treated water emanating from the reaction column had a NO₃ ⁻ contentof not more than 1 mg/liter.

EXAMPLE 2

The cathode used in this example comprised a carbon matrix, anapproximately 2 mm layer of an aggregate of carbon fibers formed on thecarbon matrix by heating rayon fibers to 800° C., and a biocatalystconsisting primarily of methane-producing microbes and acetic acidimmobilized to a thickness of about 20 μm on the carbon fiber layer.

This cathode and a carbon anode were disposed in a vessel containing 800cc of water having a HCO₃ content of 1,000 ppm and an electric currentwas applied between the cathode and anode. The cathode current densitywas 0.714 mA/cm² and the temperature of the water 35° C.

The time--vs--methane generation relationship is shown in FIG. 3.

EXAMPLE 3

An operation was carried out by following the procedure of Example 2,except that a macromolecular absorbent material about 2 mm thick andcapable of absorbing water and consequently swelling in a gel form wasused in the place of the aggregate of carbon fibers. The relationbetween the elapse of time and the amount of generated methane is shownin FIG. 4.

EXAMPLE 4

A biocatalyst-immobilized electrode according to the present inventionwas produced by forming a biomembrane containing a methane-producingmicrobe in a thickness of about 100 μm on a cathode matrix materialformed of carbon.

In a reaction vessel, the electrode mentioned above was disposed as acathode and an anode separately made of carbon was opposed to thecathode. Between the cathode and the anode of the reaction vesselsupplied with water containing 1,000 ppm of HCO₃ and maintained at atemperature of 35° C., an electric current was supplied at a cathodecurrent density of 0.357 mA/cm² for 100 hours, then at a cathode currentdensity of 0.714 mA/cm² for 50 hours, and further at a cathode currentdensity of 1.07 mA/cm² for 50 hours, to test for the amount of CH₄ andH₂ generated. The results are shown in FIG. 5. In the diagram, the markstands for the amount of CH₄ generated and the mark o for the amount ofH₂ generated.

What is claimed is:
 1. A method for treating water comprising:reducingcompounds in a water sample by applying a voltage between a firstelectrode comprising:(a) an electrical conductor; and (b) a biocatalystwhich reduces organic substances or inorganic ions immobilized on asurface of said electrical conductor, wherein said biocatalyst isselected from the group consisting of Paracoccus denitrificans,Micrococcus denitrificans, Alcaligenous, Pseudomonas, Clostridiumaceticum, Acetobacterium woodii, Methanobacterium, Enterobacter cloacal,and sulfuric acid reductases; and a second electrode comprising anelectrical conductor, wherein said electrodes are immersed in said watersample such that application of said voltage generates hydrogen at thesurface of said first electrode.
 2. A method according to claim 1,wherein said electrical conductor is a carbonaceous substance.
 3. Amethod according to claim 1, wherein said electrical conductor isporous.
 4. A method according to claim 1, wherein said biocatalyst isoverlaid with a covering material.
 5. A method according to claim 4,wherein said covering material is selected from the group consisting of:dextran, carrageenan, alginic acid, polyvinyl alcohol, a photo-linkingresin or urethane polyacrylamide gel.
 6. A method according to claim 1,wherein said biocatalyst is immobilized by way of a supporting member.7. A method according to claim 6, wherein said supporting membercomprises a fibrous substance.
 8. A method according to claim 1, whereinan organic hydrogen donor is added to said water sample.
 9. A methodaccording to claim 8, wherein said water sample is denitrified.
 10. Amethod according to claim 9, wherein said organic hydrogen donor isadded in not more than a stoichiometric amount based on the amount ofnitrate in said water sample.
 11. A method according to claim 9, whereinsaid organic hydrogen donor is hydrogen, an alcohol, an organic acid ora hydrocarbon.
 12. A method according to claim 1, wherein the pH of saidwater is maintained at an optimal value for said biocatalysts.
 13. Amethod according to claim 12, wherein said pH is maintained by addingcarbon dioxide to said water sample.